1. Field of the Invention
The field of the invention is that of transmission of digital data over a baseband serial link between a transmitter and a receiver linked by a communication channel.
The invention relates more particularly to a device for reconstructing the clock signal of the baseband serial message transmitted over such a communication channel, as well as a data transmission system incorporating such a device.
In baseband serial digital links, it is necessary, at the receiver, to extract from the received signal both the digital data transmitted serially by that received signal and the clock signal corresponding to those data.
2. Description of Related Art
Two methods are known for recovering the bit timing from the received signal.
According to a first method, the clock signal is sent by superimposing it on the data signal on the transmission channel. This first method has the disadvantage of consuming part of the transmitted power needed for the signal, as well as part of the usable passband of the transmission channel.
According to a second method, the clock is reconstructed from the transitions in the signal. It is therefore necessary to transmit signals such that the receiver can find many transitions, even for sequences of identical bits. This constraint excludes the use of NRZ (non-return to zero) coding.
Currently, clock extraction circuits are generally made based on a phase-locked loop, or PLL, which is made up essentially of a phase comparator which provides a setpoint signal to a voltage-controlled oscillator via a loop filter. Thus, a PLL achieves constant phase matching of the clock signal provided by the oscillator with the received signal by slightly altering the oscillator frequency to correct any possible phase offset.
More specifically, when the signals (clock and data) are out of phase, the phase comparator generates an error voltage. The loop filter filters this error voltage and its average voltage directly controls the oscillator. When the loop is locked, however, the two signals are in phase and no error voltage is generated.
In the case of data transmissions including long sequences of identical bits (hence with no transitions, which is the case with NRZ coding in particular), no error voltage is available and the oscillator is then liable to drift and to break the lock.
Apart from its relative simplicity, however, NRZ coding has the advantage of having a relatively limited spectrum, which makes it possible to consider its use on narrowband media because the spectrum use of the signal is then limited, or in applications requiring that the passband occupied by the serial data transmission be minimized. NRZ coding in particular proves advantageous in the case where a single shared transmission line is used for the transmission of electrical power and bidirectional data between, a transmitter and a receiver.